Field of the Invention
The present invention relates to optical systems and image pickup apparatuses. The present invention more particularly relates to an optical system suitable for an image pickup apparatus with use of a solid-state image sensor, such as digital still cameras and digital video cameras, and to an image pickup apparatus including the optical system.
Description of the Related Art
Image pickup apparatuses with use of a solid-state image sensor, such as digital still cameras and digital video cameras, have spread. With recent development of higher-performance, smaller-size, and the like of imaging optical systems, small imaging systems are rapidly spreading in particular.
Various single-focus optical systems have been proposed as an optical system for such a small imaging system. For example, Japanese Patent Application Laid-Open No. 2014-109700 discloses a single-focus optical system having a relatively long focal length, the system equipped with, in order from the object side: a front group having positive refractive power; an aperture stop; and a rear group having negative refractive power. Generally, in a telephoto lens having a long focal length, chromatic aberration occurs more frequently as the focal length is longer. Accordingly, in the single-focus optical system disclosed in Japanese Patent Application Laid-Open No. 2014-109700 for example, excellent image formation performance is implemented by arranging a diffraction grating surface in an optical path and correcting the chromatic aberration with the diffraction grating.
However, since the optical system disclosed in Japanese Patent Application Laid-Open No. 2014-109700 includes a large number of lenses, reduction in size of the optical system is not fully achieved. The optical system also has insufficient brightness as its F-number is larger than 2.8. In recent years, the optical system for small imaging systems are demanded to achieve further reduction in size. In the case of single-focus optical systems in particular, larger-aperture systems with F-number smaller than 2.8 are highly demanded.
An object of the present invention is to provide small, high-performance, and large-aperture optical system and image pickup apparatus which are suitable for a small imaging system.